Polyoxymethylene dialkyl ethers (RO(CH2O)nR) are novel blending components for clean oil product, which have very high cetane number (H3CO(CH2O)2CH3: 63, H3CO(CH2O)3CH3: 78, H3CO(CH2O)4CH3: 90, H3CO(CH2O)5CH3: 100, H5C2O(CH2O)2C2H5: 77, H5C2O(CH2O)3C2H5: 89) and oxygen content (methyl series: 42%-49%, ethyl series: 30%-43%). When polyoxymethylene dialkyl ethers are added to the diesel oil at 10%-20%, it significantly improves the combustion characteristic of the diesel oil, increases the thermal efficiency, and greatly reduces the emission of NOx and soot. Therefore, polyoxymethylene dialkyl ethers are considered as very promising environment-friendly diesel blending components. U.S. Pat. No. 7,235,113 discloses that when 15% of H3C(OCH2)3-6OCH3 is added to the diesel oil, the emission of NOx, particles, and hydrocarbon of the exhaust achieves Euro V standard.
EP 1505049 A1 and U.S. Pat. No. 6,534,685 by Snamprogetti S.P.A. Corporation disclose a process for synthesizing polyoxymethylene dimethyl ethers (DMMn) and recycling the materials, where the acetalation reaction between polyformaldehyde and methylal under the catalysis of liquid acid produces DMM2-5, the reaction solution is absorbed via silica gel column to remove the liquid acid catalyst, the treated reaction solution enters the rectification column, the light components (trioxymethylene, DMM1-2), the products (DMM3-5), and the heavy components (DMM≧5) are separated using two-stage rectification process, and the light components and the heavy components are recycled to the reactor for reuse. PCT Publication WO 2006/045506 A1, Canadian Patent Application Publication CA 2581502 A1, U.S. Patent Application Publication Nos. 20070260094 A1 and 20080207954 A1, by BASF Corporation, disclose similar catalysts and products separation process in which the reaction between trioxymethylene and methanol catalyzed by the liquid acid produces DMM1-10 and byproduct water. The reaction solution is subject to the absorption through the packed column charged with the anion exchange resin to remove the acid and water, the treated reaction solution enters the rectification column, and the products DMM3-4 are separated through three-stage rectification, where DMMn with n≦2 and n≧5 are recycled to the reactor for reuse. The above reaction solution separation process employs large amount of absorbents, thereby resulting in high energy consumption, and the catalyst cannot be reused.
U.S. Patent Application Publication No. 20080207954 A1 by BASF Corporation discloses a process of producing DMM1-5 through the reaction of methanol and formaldehyde in an aqueous solution catalyzed by a liquid acid or a solid acid, where a reaction rectification technique is employed for separating the crude products (DMM1-5, raw materials, and water) and the catalyst, and the crude products are separated into the light components (DMM1-2 and unreacted raw materials), the products DMM3-4 (containing water), and the heavy components DMM>4 by a multi-stage rectification process. But in the actual operation of the process, it is difficult to separate methanol and DMM3-4 due to the azeotrope of methanol, water, and DMMn. Meanwhile, the good miscibility of methanol, water, and DMM3-4 causes phase separation more difficult.
Recently, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, in U.S. patent application Ser. No. 13/154,359 and published as U.S. Patent Application Publication No. 20110288343 A1, UK Application No. 1108697.2 and published as GB 2489534A, U.S. patent application Ser. No. 13/164,677 and published as U.S. Patent Application Publication No. 20110313202 A1, and UK Application No. 1110391.8 and published as GB2483325A, discloses a method of synthesizing DMM1-8 through the reaction of trioxymethylene and methanol catalyzed by ionic liquids, where the reaction solution is subjected to flash evaporation, film separation, and phase separation to separate the light components (DMM1-2, a part of water, unreacted raw material), the crude products DMM3-8, and catalyst. In order to achieve the purification of the products DMM3-8, small amount of water and catalyst contained in the crude products need to be removed by absorption with silica gel or anion exchange resins, thus, the recovery rate of catalyst of the process is relatively low.